Strenuous, Fatiguing Exercise: Relationship of Cortisol to Circulating Thyroid Hormones

The purposes of this study were to determine how strenuous, fatiguing running exercise affects: 1) selective thyroid hormones, and 2) the relationship of glucocorticoid responses to such exercise withthyroid hormones.materials and methods: well-trained subjects (n=12) performed a treadmill run at individual ventilatory threshold (74±8% of maximal aerobic capacity) until volitional fatigue (68.3±12.3 min).blood samples were taken before exercise as a resting ba seline (bo), at fatigue (fg), 90-minutes into recovery (90mr), and 24-hours into recovery (24hr). blood was analyzed for free t3 (it3), freet4 (ft4)/ thyroid-stimulating hormone (tsh), and cortisol.results: significant increases in it3 and tsh concentrations between bo and fg (it3= 1.70 pglml vs, 2.08 pglml; tsh=1.69piu/ml vs. 2.43pill/ml, p<0.01) were observed, but by 24hr, significant decreases from bo were present (ft3 and tsh=1.48 pglml and 1.20 piujml, respectively; p<0.05). fg ft4 demonstrated a nonsignificantincrease from bo (fg ft4=1.84 ngldl) but by 24hr ft4 was significantly lower than fg (24hr ft4=1.67 ngldl, p<0.01). fg cortisol levelsincreased significantly from bo (476.1 to 843.9 nmolfl, p <0.01) and remained elevated at 90mr (892.2 nmol/l) but returned to baseline by 24hr.spearman correlation analysis yielded a significant negative correlation between fg cortisol and 24hr tsh (r= -0.65/ p<0.05). a strong trendwas also noted between fg cortisol and 24hr fh (r = -0.55/ p<0.07).conclusion: these findings suggest that exhaustive exercise decreases the level of selective thyroid hormones by 24 hours into recovery, andthat cortisol levels after fatiguing exercise are negatively related to circulating tsh at this point into recovery.